KR101046287B1 - Substrate Surface Treatment Method - Google Patents

Abstract

The present invention relates in general terms to the treatment or processing of substrate surfaces. In particular, the invention relates to processes for modifying the surface of silicon wafers.

Description

Substrate surface treatment method {METHOD FOR THE TREATMENT OF SUBSTRATE SURFACES}

The present invention relates to a substrate surface treatment method. The present invention more particularly relates to a method for modifying the surface of silicon wafers.

During the production of silicon pieces, silicon plates, or wafers for the semiconductor and solar cell industries, wafers require mechanical or chemical processing steps to impart desired size and production characteristics. The following text shows the processing steps for manufacturing a conventional solar cell according to the prior art.

First, silicon ingots are divided into pieces, also known as wafers, using a wire saw. After the wafers are divided, the wafers are cleaned to remove what is known as sawing slurry. This is a wet-chemical saw damage etch that utilizes suitable chemicals, especially alkalines, to remove the defect-rich layers resulting from the cutting process. Is usually done. Wafers are then cleaned and dried.

Wafers or substrates are generally monocrystalline or monocrystalline silicon wafers p-doped with boron. One side of the silicon wafers is n-doped to produce the pn junction required for the solar cell to function. This n-doping is generally carried out by phosphorus doping. In this method, the substrate or silicon surface is altered by the incorporation of phosphorus atoms and the phosphorus source is generally used in gas or liquid-past composition state. do. After proper incubation or coating of silicon wafers in the gas or with the composition, phosphorus atoms are usually diffused, accumulated or incorporated into the silicon surface by heating at 800-1000 ° C. After this phosphorus doping, the silicon plate has an n ⁺ type doped layer with phosphor that reaches some micrometer thickness.

One problem with this surface change is that in general, not only the desired surface (top), but also the opposite side (bottom) and in particular the peripheral edges of the silicon wafer are changed or doped by the treatment and the edges are electrically conductive, so that in later use It leads to the risk of short circuits. However, additional doping of the underside, such as, for example, affected by vapor phase doping, indicates that n ⁺ doping of the underside or back surfaces of the plate is referred to as "aluminum after surface field." (aluminum back surface field) ", for example, for sequential contact-coupling of solar cells, which is then generally changed to P ⁺ doping, as required, which is acceptable in many cases. have. However, wafers processed in this manner always have edges containing phosphorus atoms and are therefore electrically conducting, which results in silicon wafers having the above drawback for the risk of short circuits formed with sequential use without further processing. Will lead.

Various methods have been developed as prior arts to eliminate this problem. As an example, the problem of electrically conductive edges is solved by edges being ground away mechanically. However, grinding, such as sawing, can cause defects in the crystal structure, leading to electrical losses. However, an important drawback of this process is the significant risk of breaking sensitive wafers.

Moreover, it is proposed that the existing conductive layer of the bottom or back surface should be interrupted at the outer area or edge by the operation of the laser beam. However, edge insulation by lasers does not yet have a stable method, and particularly addresses the problems with respect to the throughput and process automation that can be achieved. Moreover, there is a risk that, for example, the efficiency of the cell processed correspondingly with the sequential processing steps will have the opposite effect by the accumulation of combustion products formed during the post-treatment on the wafer surface.

Finally, it is proposed that a number of plates are stacked and the plate stack is etched by the plasma. Edge insulation by plasma requires wafers to be stacked on top of each other. Both stacking and coordination of stacks takes place in a manual or automated manner, involving very high levels of spending on equipment. As a result, the method in stacks is unique both in terms of batch production in which wafers are transported to a process carrier and in the case of inline production where wafers are passed through various processing steps, such as on conveyor belts or rollers. Always include interrupting or reorganizing the production flow. Moreover, complicated adjustments mean that the wafers are once again exposed to an increased risk of disconnection.

Further methods in which only the edges are processed are proposed in DE 100 32 279 A1. DE 100 32 279 A1 proposes methods for chemical protection of edge defects in silicon solar cells by etching edge defects. For this purpose, an etchant is applied to the edges of the silicon solar cells using a felt cloth infused with the etchant.

More methods known from the prior art solve the problem of electrically conductive edges by removing the edge conductive layer and one side of the substrate by etching in an acid bath. By way of example, DE 43 24 647 A1 and US 2001/0029978 A1 describe multistage etching methods in which the substrate is completely submerged in an acid bath. It is in each case only the back surface and edges of the substrate to be etched here, so the front surface of the substrate must be protected by an acid-resistant photoresist or mask.

In particular, the etching method described in DE 43 24 647 A1 and US 2001/0029978 A1 not only requires time-consuming, but also the use of additional materials, as special work steps require the application and removal of the protective layer. Require. In particular, the application and removal of the protective layer involves the risk of substrates being treated to be adversely affected. If a defect or loss occurs in the applied protective layer, there is a risk of damaging the entire surface of the substrates during etching, making the substrates unusable.

Therefore, all these methods described in the prior art help to separate the two surfaces (top and bottom) with respect to their electrical conductivity, but they in some cases include serious problems of the type described above.

It is therefore an object of the present invention to provide a method for treating one aspect of silicon wafers that can be performed without prior art processing steps, including the protection or masking of untreated entire surfaces or top surfaces, The method may be carried out preferably on a production line.

In accordance with the present invention, it can be seen that it is possible to selectively process only one of the two surfaces of the substrates. In this way, the treatment of one side includes, for example, etching, coating, or cleaning one of the two surfaces. According to one embodiment, only the top or bottom surface of the corresponding substrate, such as, for example, a silicon wafer, can be changed by etching, so that the problem of the formation of short circuits is eliminated in a simple manner. For ease of understanding, the following text shows the etching of a surface as an example of treatment for one side of a substrate.

According to a preferred embodiment, the method according to the invention is carried out at the bottom of the substrate, in particular silicon wafers (if desired to include peripheral edges) which are immersed in an etching liquid located in a liquid bath. , As part of the ongoing treatment.

It should be noted that the method according to the invention is particularly suitable if desired or necessary for treatment of only one side of the substrate. According to a preferred embodiment, the method according to the invention provides a substrate, preferably in the form of a silicon wafer, so that a single-sided etching is supplied after the phosphorus-doped to remove the phosphorus-doped layer. It is in contact with a liquid composition where only one side of the silicon wafer comprises, for example, oxidic acid, preferably HF with NaOH, KOH, HNO ₃ , O ₃ , and / or HF with an oxidizing agent. It is influenced by the fact that it occurs completely or partially.

For this purpose, the silicon wafer is oriented substantially horizontally, and the surface to be etched is immersed in the etching liquid located in the bath. The distance between the etch liquid and the underside of the silicon wafer is selected so that the underside of the silicon being etched (if desired to include peripheral edges) is wetted, but the opposite side is not.

Since phosphorus glass etching is generally carried out by wet-chemical means, and then edge insulation according to the invention is carried out in the same apparatus, this etching step is preferably carried out after phosphorus doping. Note that this can be done directly, saving space and inexpensive resolution. However, it will be apparent to one skilled in the art that the steps according to the invention are also performed at other times. The only important factor is that the etching according to the invention must take place before the application of metal contacts to the back surface or underside of a given substrate.

According to one preferred embodiment of the method according to the invention, both one side of the substrate and the peripheral edge of the substrate will be treated in the outlined manner.

According to one embodiment, the substrates are lowered into a liquid bath containing a liquid composition, in which case the range of lowering them is a function of thickness, weight, surface properties of the substrate, and surface tension of the liquid composition. It is easy to set up for an experienced person. Furthermore, according to the present invention, for example, by the correct setting for the level of the processing liquid bath, preferably, not only the bottom but also the edge to be processed are possible.

The method according to the invention can be carried out in other ways as well as lowering into the bath if it is used to ensure that it is actually only one side, and also use edges that are wetted by the etchant and the resulting change. Will be apparent to those skilled in the art. By way of example, it is possible to provide two containers of different sizes, in accordance with another embodiment a smaller container comprising a liquid composition and a larger container enclosing it. The smaller container is filled with liquid and flows in by engaging with the larger container. For example, this supply of liquid occurs continuously by the pump, preferably some amount of nickname solution, pumped back from it into the inner tank (smaller container), always with the outer tank (larger container) It can be set up in such a way that it flows in. Pumping the liquid composition means that the liquid level is always slightly higher than the peripheral edge of the smaller vessel, and in particular the difference between the height of the container edge, which depends on the liquid level and the surface tension of the etching medium, is used. . By using this configuration, it is possible for wafers that are horizontally moved and processed across the liquid as part of the production line to be easily wetted without any possibility that the underside of the wafers is encountered facing the side of the smaller inner container.

In another embodiment, it is possible to use dipping processes, in which case the liquid level in the bath is sufficiently greater than the underside of the wafers so that if used with edges, it will only get wet at the lowest position of the dipping curve. Is set low.

The wet or processing of a single side of the proposed substrate in accordance with the present invention of the embodiments described above is accomplished or assisted in a variety of ways, with the fundamental differentiation being between active (direct) or passive (indirect) wet. Will be in.

In the description of the present invention, active or direct wet will be understood to mean that the desired treatment of one side of the substrate is made reliably immediately by the substrate passing through the processing liquid. According to the invention, this requires the level of the substrate that can be treated such that the underside of the substrate is at least for a short period of time below the maximum level of the processing liquid. By way of example, in terms of active wetness, the substrate can be lowered into the liquid, or the liquid level of the tank can be raised completely or partially, and the present invention also provides a combination of lowering the substrate and raising the liquid level. Include.

As an example, the surface of the bath may be locally raised by a correspondingly disposed and directed liquid inlet below the bath surface at the position where the substrates are inserted into the bath. In addition, the bath surface may be partially raised by blowing bubbles of gas under the substrate, for example using compressed air, to ensure wetness of the bottom of the substrate.

In contrast, in the context of the present invention, passive or indirect wet means that the bottom of the substrate to be treated is above the liquid level throughout the pretreatment period so that wet is in contact with the liquid and is responsible for the wet of the bottom of the substrate. It will be understood that some of the components can only be made indirectly by contact with the liquid and in charge of the wet bottom of the substrate. In this description, the hygroscopic properties of the surfaces of the silicon wafers ensure that the entire surface is wetted within a very short time even with partial wet of the underside by the part, so that the substrate surface being treated is wet. It should be noted that it needs to be wetted completely (over the entire surface) or only partially through contact with the component in charge.

With regard to parts that may be provided indirectly wet, it is noted that the parts form part of the conveyor system described above or are disposed in a bath such that the parts may at least partially protrude out of the liquid or extend out of the liquid. It should be noted. Thus, parts which are fixed according to the invention, which are rotatable or movable in the vertical direction are equally suitable. The surface properties and / or morphology of the part, for example by using a capillary effect, results in the area of the part intended to come into contact with the bottom of the substrate so that the surface to be treated is wetted without contacting the bath itself with the substrate itself. It is preferable. By way of example, the part would be a wet roller that rotates in the liquid of the liquid bath and, as a result of the rotational movement, absorbs the etch liquid and then wets the substrate base located at that level by this etch liquid. However, as already explained, it is surprisingly enough that even the punctiform in contact with the underside of the substrate is sufficient to ensure the wetness of the entire substrate, and according to the invention, the tables, pins, or pistons are (vertically movable). It is also possible to use parts of other structures, such as rams.

In principle, the use of a conveyor system for guiding substrates to be processed as part of the process according to the invention allows both active or passive wet. In the case of active wet, the substrate to be processed is guided through the liquid, while passive wet is achieved by correspondingly constructed components of the conveyor system.

Suitability of examples for the conveyor system according to the invention is described in detail below with additional reference to FIG. 1.

According to one embodiment of the invention, the substrates are placed in a conveyor system, for example a roller conveyor system. In this case, the substrates are conveyed with the help of many conveyor rollers 1 placed back and forth and laid horizontally. In view of the active wet defined above, the individual conveyor rollers are wetted by direct contact of the underside of the substrate with the bath surface, in each case the upper edge of the rollers is at the level of the bath surface, i.e. the top of the liquid. It is preferably arranged in such a manner in the liquid bath, which is at approximately the same position as the edge. In this case, a meniscus will be formed at the substrate edges. The interaction of gravity and surface tension then pulls the substrate down and keeps the substrate in firm contact with the rollers without rising. This allows for controlled and limited conveyance of substrates using a roller conveyor system.

In this respect, it is important that the height of the bath can also be set very accurately for the conveyor system so that the bottom and, if appropriate, the edges of the substrate can be wetted without the top surfaces of the substrates being wetted. In addition, the configuration of the conveyor system must allow for contact between the substrate and the liquid in the liquid bath.

According to a preferred embodiment, there are at least two support elements 3, which will preferably be arranged in the conveyor roller in the region of the two grooves 2, which are present in the conveyor roller 1. The distance between the support elements is preset by the width of the substrates to be processed. In view of active wetness, the above description relating to the positioning of the conveyor rollers in this embodiment applies to the supporting elements.

In view of the passive wet defined above, the conveyor rollers themselves or the supporting elements are responsible for the complete or partial wet of the substrate undersides.

The conveyor roller preferably has a two part structure, comprising an axle element and at least one track element surrounding the axle element. The shaft element will simply function as a ballast or ballast bearing. It is probably a bearing axis. Axial materials that are not brought into contact with the material being transported or the erosive chemical environment under any circumstances can be readily selected based on mechanical and thermal aspects. According to the invention, the axial material is rigid in terms of flexurally rigid. In contrast, the track element allows some thermal tolerance due to the stable bearing axis. An important element of the material is that it does not react to the piece material or ambient medium. A flexurally rigid bearing axle ensures that the material to be conveyed is held reliably on a straightened straight line in a direction perpendicular to the conveying direction. As a result, the conveyor rollers operate simultaneously over their entire length, which is particularly important in the case of fairly wide conveyor rollers with many conveyor tracks and for flat materials to be transported which are susceptible to breakage.

In a preferred embodiment, the shaft element is made of carbon fiber composite. Carbon fiber composites have high thermal or mechanical stability and are therefore particularly stable for use with bearing axes used at varying temperatures.

In a preferred embodiment, for the medium used to treat the material being conveyed, the bearing axis is encapsulated, for example by sealing the rings. In accordance with the invention, the medium, which is a wet-chemical liquid bath, is then only in contact with the outside of the track elements and the liquid medium may be present inside the track elements, the bearing axis, or between the bearing axis and the track elements. It does not penetrate into any fixed components. The seal will be liquid-tight, or designed to the extent that it is gastight, so that no harmful vapors also penetrate into the interior of the track elements.

The track elements can be assembled to any desired length, and the conveyor roller will comprise an axis with any desired many track elements, for example. Conveyor roller manufacturers or suppliers can be very flexible in meeting customer needs without the use of complex stock management. A track element is a mass-produced article, which reduces production costs, as it can be used with conveyor rollers of any length.

The track elements can, for example, be plugged together, screwed together, joined using a clip or welded together.

In a preferred embodiment, the substrate to be treated is, as mentioned, a support element having static friction properies suitable for the workpiece, in which case the support elements can be used for passive wet as well as transport. Is actually supported on the field 3. Such support elements will also be thermally or chemically stable. The use of O-rings made from fluorinated rubber proves to be suitable for the production of solar cells. Since the diameter spanned by the support element is larger than the rest of the track element, the material to be transported only experiences punctiform contact and, if appropriate, wet. This, unlike linear contact, is suitable for the material being conveyed and at the same time ensures good bonding to the surrounding medium.

In a more advantageous embodiment, the track element is made of plastic. It is known that plastics are easy to handle and offer a wide range of other properties that are selected depending on the use of the conveyor roller and the placement location. As an example, it proves suitable for using polyethylene, polyfluoroalkoxide, or polyvinylidene fluoride. These materials are thermally stable up to 80 ° C., weldable, have some chemical stability, do not cause any metal contamination, and are relatively unupratable.

An advantageous refinement of the invention lies in the track elements being driven, ie the drive is applied to the bearing axis and then transmitted directly to the track elements from the latter rather than to the track elements. It is quite possible to be. Conveyor rollers with track elements of this type can in particular be assembled to form a synchronous conveyor system. Optimum traction is transmitted to the material being conveyed.

In one embodiment, the first edge component of the assembled series of track elements has a means for transmitting a driving force and the second edge track element has a means for a rolling bearing. The driving force can be transmitted to the conveyor roller via the coupling component, which is fitted to the drive shaft. The coupling component also has a means for holding the bearing axis. When the conveyor roller is removed from the transport position, the second edge track element must first be released from the bearing, the entire conveyor roller must be rotated relative to the coupling component, and then the conveyor roller must be removed from the coupling component.

Means for rotating the bearing will include an upper half-shell fixed to be released and a lower half-shell fixed to the wall of the conveyor system and used to support the conveyor roller. .

The width of the track element corresponds to at least suitably the width of the workpiece to be carried, such that the wide side of the workpiece is located in only one track element. Each track element receives only one workpiece if possible, ie the width of the track element and the material to be conveyed are as close as possible.

In an advantageous embodiment of the conveyor roller, the stationary ring is fitted to the bearing axis, the inner diameter of the track element being smaller than the diameter of the stationary ring in at least one position. The stationary ring therefore hinders the possibility of a track element that performs much greater movement with respect to the bearing axis. This is particularly important in the event of temperature change, which can cause the material and track elements of the bearing axis to move in proportion to each other if they expand differently.

The retaining ring is preferably made of metal as the metal can be successfully bent into the shaft and clamped in place.

For use at other temperatures, slight variations in length do not adversely affect the stability of the conveyor roller as a whole. Therefore, in a more advantageous embodiment of the invention, the track elements comprise a compensation crease where the temperature expansion is compensated. The compensation crises generally comprise an inner hollow convexity in the material of the track element, which absorbs the temperature-induced material expansion by stretching in the direction of the length. If the compensation cris is not located between the support points of the material being conveyed, the stability of the support is stable even in the case of temperature-induced changes in length. In addition, if the track elements are fixed to the bearing axis in each case, the straight line characteristics of the track also remain.

The constant guidance of the material conveyed is particularly successful when the conveyor rollers according to the invention are assembled to form a conveyor system.

In a preferred refinement of the conveyor system, each conveyor roller is driven. In this case, each conveyor roller is susceptible to the same transmission force, and therefore also to the same rod.

The conveyance guidance provided by the proposed conveyor system according to the invention allows for a high throughput and is very smooth for materials carried at the same time, in particular suitable for use in view of the treatment according to the invention. Do.

It should also be noted that other embodiments of the conveyor system without conveyor rollers are possible. By way of example, the substrates may also be carried on a rotating belt, chain or also cords. Better transport options in the conveyor system are formed by a traveling bar. The system uses two or more bars where the substrates alternately carry the substrate forwards. When the first bar moves forward, the second bar moves backwards. In this case, the second bar is placed deeper in the bath and is not in direct contact with the substrate. The first or upper bar is raised so that the substrates reach the end of its possible conveying movement and the second or lower bar reaches its start, so that the substrates contact all the bars. At the same time as the lower bar performs the movement forward, the upper bar is then lowered and can therefore go to be the starting point of the bath.

In traditional designs of this type of bar conveyor system, the bars are installed on a rotating shaft with an eccentirc, ie they always move up or down. However, to ensure processing on one side of the substrates, in terms of active wetness, the substrates should always be kept at the same height. However, as mentioned above, the change in terms of passive wet corresponds to a traditional bar conveyor system and its use is immediately apparent to those skilled in the art having the knowledge of the present description.

Therefore, the method according to the invention is advantageous to be carried out especially in once-through installations, since no additional adjustment steps are required for wafers in terms of this type of inline production. In addition, since the surface / edge insulation according to the present invention is performed together by oxide etching in the same equipment, the processing sequence can be made simpler and less expensive. In addition, by using the method according to the invention, it is also possible to realize cell concepts in which the back surface of the cell does not have an entire area "aluminum back surface field (AlBSF)". Since in the method according to the invention such doping is no longer required to be compensated by forming AlBSF to form a p-doped zone, the n-doped layer of the back surface of the cell is completely removed. This leaves more options open regarding the setup of the back surface of the cell, simplifying the realization of the cell concept without AlBSF.

Depending on the particular treatment (continuous or discontinuous), the liquid composition requires additives, for example to avoid or reduce the size of gas bubbles, in which case this type of additive has special requirements. It can be selected by a person skilled in the art without difficulty on the basis. Particularly in the once-throw process, it should be ensured that, if a suitable additive is selected, the wafers cannot obtain excess buoyancy as a result of the possible formation of bubbles. As a result, according to a preferred embodiment the etching solution comprises at least one additive which substantially binds the gas formed during the chemical reaction such that the formation of bubbles on the underside of the wafers is substantially suppressed.

The treatment according to the invention is not only required for the electrical insulation of the two sides of the wafers or the solar cells, but also for the treatment of one side of the substrate with the liquid medium, for example, as in the case of cleaning or coating, for example, It should be noted that it is also suitable for carrying out other wet-chemical treatments.

Claims (28)

A method of wet-chemical treatment of one side of a silicon wafer using a liquid bath, The silicon wafer is placed on a conveyor means during the processing so that the entire surface of the underside to be treated is passed through or carried over the liquid in the bath, The conveyor means is disposed in the bath, In addition, the wet-chemical treatment method for one side of a silicon wafer using a liquid bath, wherein the top surface of the silicon wafer that is not processed is always above the level of the liquid. delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete The method of claim 1, Wherein said top surface of said silicon wafer is not protected during said processing. The method according to claim 1 or 18, Wherein the edges of the silicon wafer are also processed. The method according to claim 1 or 18, Wherein said conveyor means is provided in the form of a belt or a roller. The method according to claim 1 or 18, Wherein said silicon wafer is continuously processed in a once-through process. The method of claim 21, And a bottom surface of the silicon wafer is lowered into the bath, wherein the wet-chemical treatment of one side of the silicon wafer using the bath. The method according to claim 1 or 18, Wherein the silicon wafer as part of a production line is passed horizontally over or above the processing solution in the bath, using a bath to wet-chemically process one side of the silicon wafer. The method according to claim 1 or 18, Wherein the used bath is a tank whose peripheral edge is lower than the level of the liquid. The method according to claim 1 or 18, Wherein said processing is an etching step. The method of claim 25, Wherein said etching is performed in a liquid composition comprising KOH, HNO _3, HF with O ₃ , and / or HF with an oxidant. . The method of claim 26, Wherein the oxidant is an oxidizing acid, wherein the wet-chemical treatment of one side of the silicon wafer using a liquid bath. The method of claim 26, Wherein said liquid composition comprises at least one additive for combining gases formed during said etching.

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